Thin-film technologies are currently being developed for the purpose of reducing the cost of semiconductor devices, in particular photovoltaic (PV) cells. Whereas conventional solar cells are made of slices of solid crystalline silicon wafers, which have thicknesses of typically a few hundred microns, thin-film materials can be directly deposited onto a substrate to form layers of ˜2 μm or less, resulting in lower material as well as lower manufacturing costs. Moreover, thin-film technologies allow for monolithic integration, i.e. the in situ creation of electrical connections, which further reduces production costs.
Thin-film materials include cadmium-telluride (CdTe), copper indium diselenide (CIS) and variants thereof, amorphous silicon, and polycrystalline silicon (<50 μm). In recent years, technical progress has occurred particularly in thin-film technologies based on CdTe and CIS. Both materials have high absorptivities, so that most of the incident radiation can be absorbed within 1-2 μm of the film. Used as the absorber layer, in which incoming photons create electron-hole pairs, these materials can be paired with, for instance, a layer of CdS, to form heterojunctions, and sandwiched between front and back contacts to form a solar cell.
To gain widespread acceptance, thin-film PV cells must exhibit high conversion efficiencies of photon energy to electric current, and operate reliably in an outdoor environment over many years, ideally no less than 30 years. Technologies based on CdTe and CIS have demonstrated long-term stability; however, performance degradation has also been observed. Efficiencies of current thin-film devices reach 65% of the theoretical maximum (75% in the laboratory), still lagging behind some monocrystalline silicon and GaAs cells, which have demonstrated 90% of their ultimate achievable performance. Improvements in efficiency of thin-film technologies can be achieved through multijunctions and graded materials. For example, studies on CIS have revealed that doping with gallium, to form compounds referred to as CIGS and exhibiting gradients in the concentrations of Ga and In, lead to better efficiencies.
The complexities of thin-film technologies, which are essential for high efficiencies, adversely affect cost and manufacturability, establishing a need for improved techniques—in particular low-cost techniques amenable to practice with off-the-shelf equipment. Challenges to the development of low-cost and reliable CIGS and CdTe devices include the standardization of equipment for layer deposition, absorber layers having thicknesses less than 1 μm, and control of film uniformity over large areas.